1. Field of the Invention
The present invention relates to an optical fiber which can be suitably used as an optical transmission path and a dispersion compensator.
2. Description of the Related Art
FIG. 10 is a cross-sectional view of an optical fiber including microstructures which has been known conventionally. As shown in FIG. 10, this optical fiber has a cross-sectional structure which has a large number of voids (vacant holes) 62 in a material made of silica glass 61. A central portion in cross section having no voids 62 constitutes a core region 63 and a portion surrounding the core region 63 and including a large number of the voids 62 constitutes a cladding region 64.
The principle of optical confinement of the optical fiber including such microstructures is explained using a concept called effective refractive indices qualitatively (for example, T. A. Birks et al. Optics Letters Vol. 22 p.961 (1997)). Due to the presence of the microstructures, in a strict sense, the refractive index should exhibit a complicate distribution in the core region 63 and the cladding region 64. However, on the assumption that the optical waveguiding characteristics can be approximated by replacing each region with a homogeneous medium, the refractive index of this uniform medium is called the effective refractive index. The effective refractive indices neff satisfy a following equation.                                           (                                                            f                  1                                                  n                  1                  2                                            +                                                f                  2                                                  n                  2                  2                                                      )                                -            1                          ≤                  n          eff          2                ≤                                            f              1                        ⁢                          n              1              2                                +                                    f              2                        ⁢                          n              2              2                                                          (        1        )            
where, n is the refractive index and f is the volume fraction. Further, a suffix 1 represents silica glass and a suffix 2 represents air. With respect to the volume fraction, f1+f2=1 holds. Usually, since n1 greater than n2, the both side members in the equation (1) become smaller corresponding to the increase of f2. Accordingly, the effective refractive index of the cladding region 64 including a large number of voids 62 becomes smaller than the effective refractive index of the core region 63 so that the light confinement is realized in the same manner as in the usual optical fiber.
Such a model of the effective refractive indices is considered to be reasonable in a case that the light wavelength is long compared to the scale of the microstructure. However, as the light wavelength becomes shorter, the light is locally concentrated at portions having the high refractive index and hence, although the effective refractive indices are elevated, simultaneously, it is considered that the assumption that the structure having the refractive index distribution can be replaced by the uniform mediums will lose the validity.
On the other hand, an optical fiber having a greater negative dispersion than such an optical fiber is disclosed in U.S. Pat. No. 5,802,236, for example. Although this optical fiber has the above-mentioned microstructures, the optical fiber is characterized in that a cladding region is constituted by an inner cladding region and an outer cladding region and the inner cladding region has the effective refractive index which is smaller than the effective refractive index of the outer cladding region.
A method for fabricating optical fibers having the above-mentioned microstructures is disclosed in Optics Letters Vol. 21 p. 1547-1549 (1996), for example. That is, a silica tube is ground such that an outer diameter becomes a hexagonal column and then the fiber is drawn to prepare a silica capillary tube and the silica capillary tubes are bundled in a hexagonal lattice arrangement to form a tube bundle. Here, the capillary tube disposed at the center of the bundle is replaced with a silica rod having no voids so as to form a core. An optical fiber having microstructures is obtained by drawing such a tube bundle.
Here, it is known that, at the time of drawing an optical fiber having such microstructures, the relative void diameter, i. e. the void diameter relative to the fiber dimension, shrinks due to the influence of the surface tension. To cope with this phenomenon, in the above-mentioned U.S. Pat. No. 5,802,236, a method in which one end of the voids extending along its axis is sealed and the fiber is drawn from the other end thereof so as to elevate the inner pressure of the voids is disclosed.
Further, in the above-mentioned Optics Letters, a technique to control the relative void diameter by controlling the temperature of a furnace at the time of fiber drawing is disclosed.
The optical fiber having microstructures is provided with various characteristics such as a large effective core area, a low bending loss or the like in response to the distribution of the microstructures in the core region or the cladding region. To determine the characteristics of the optical fiber in response to the distribution of microstructures, it is necessary that the mean refractive index distribution in cross section can be determined as desired. Further, to obtain the characteristics such as the large dispersion or the like, it is necessary to broaden the range of the value of the mean refractive index which can be realized. However, the prior art has following problems.
To realize the mean refractive index distribution by controlling the void diameter, it is necessary to form a structure where a plurality of voids having different diameters are present in the same cross section of a fiber. However, whichever method is selected from a method for sealing one end of voids and a method for controlling a furnace temperature, with respect to a plurality of voids having different diameters, it is difficult to realize the state that the diameters of respective voids have desired values. This is because the relative void diameter changes during fiber drawing and the amount of the change depends on the initial value of the relative void diameter in addition to the fiber drawing conditions. For example, the surface tension, which decreases the diameter of the voids, increases corresponding to the decrease in the void diameter. In such a conventional method, it is necessary to design the distribution of the void diameter in cross section of the preform such that the distribution of the void diameter in cross section of the fiber after fiber drawing becomes a given distribution. Such a design requires knowledge on the dependency of the change amount of the relative void diameter on its initial value and fiber drawing conditions. Accordingly, the method is extremely time-consuming and cumbersome.
Further, although the mean refractive index of the region including a microstructure is a function of the ratio between the inner diameter and the outer diameter of the silica capillary tube constituting the microstructure, it is difficult to prepare a silica capillary tube which has an extremely large or small ratio between the inner diameter and the outer diameter. Accordingly, the range of value of the mean refractive index which can be realized is also limited. When the ratio of the inner diameter to the outer diameter of the silica capillary tube is large, the strength of the capillary tube is reduced and hence, it is difficult to form voids without generating ruptures. Further, when the ratio of the inner diameter to the outer diameter is small, a fine boring instrument becomes necessary and this pushes up the fabrication cost.
Further, in controlling the void diameter, it is necessary to prepare a plurality of boring instruments corresponding to a plural kinds of void diameters and this becomes a cause of the increase of the fabrication cost.
Further, in performing the fusion splice of the optical fiber having microstructures with another optical fiber, there is a possibility that in the vicinity of an end surface of the optical fiber, the material composing the fiber is fused and hence, voids are collapsed. Since the difference in the effective refractive index between the core region and the cladding region decreases at portions where the voids are collapsed and hence, the light confinement into the core region is weakened and the light leaks to the outside of the core region so that the fusion splice loss is remarkably increased.
The present invention has been made in view of the above circumstances to provide an optical fiber which can facilitate the fabrication of optical fibers and the splice with other fibers and can make abroad range of mean refractive index available.
To achieve the above, in an optical fiber according to the present invention where the optical fiber consisting of a core and a cladding surrounding the core, and a plurality of regions constituted by sub mediums having refractive indices different from the refractive indices of the main mediums which constitute the core and the cladding are arranged in cross section, characterized in that at least one of refractive indices of the main mediums and the sub mediums are changed in a radial direction.
The main medium is made of a material which can constitute the optical fiber by itself such as silica glass, for example, and the sub medium is made of a material such as gas or liquid, for example, which cannot constitute the optical fiber by itself.
In the optical fiber having such structures, it becomes possible to adjust the mean refractive index of the region such that with the use of a material having a high refractive index as the main medium or the submedium, the mean refractive index of the region can be increased, while with the use of material having a low refractive index as the main medium or the sub medium, the mean refractive index of the region can be decreased. In the optical fiber according to the present invention, by a proper design of the refractive indices of the main mediums and the sub mediums, the mean refractive index distribution in cross section of the optical fiber can be formed without adjusting the areas of the sub mediums per unit cross-sectional area.
While the shapes of the sub-medium regions are liable to change unintentionally during the fiber drawing, the refractive indices of the materials hardly change. Accordingly, by adopting the constitution of the present invention, it becomes easy to realize a desired mean refractive index distribution in cross section of the drawn optical fiber compared with the prior art. Further, in the prior art which controls the void diameters, to increase the mean refractive index, the void diameters must be decreased. However, since there exists a limit in terms of processing technique with respect to the decreasing of the void diameters, the range of the mean refractive index which can be realized has been narrow. On the other hand, according to the present invention, the mean refractive index can be decreased by decreasing at least one of material refractive indices of the main mediums and the sub mediums, the mean refractive index can be increased by increasing those material refractive index whereby the range of the mean refractive index that can be realized can be broadened.
Further, by properly forming the distribution of refractive indices of the main mediums and the sub mediums in the radial direction, even when the voids are collapsed at the time of a fusion splice, due to the fusion of the material in the vicinity of an end surface of the optical fiber, the optical fiber can maintain the optical waveguiding characteristics. Accordingly, compared with a case where the refractive indices of the main medium and the sub medium are uniform in the radial direction as in the case of the prior art, it becomes possible to reduce the fusion splice loss so that the splicing operation is facilitated.
It is preferable that the refractive index of the main medium consisting the cladding region is changed in the redial direction. Further, it is preferable that the cross-sectional areas of the sub-medium regions are substantially uniform in the same transverse cross section. Due to such a constitution, it becomes possible to change the mean refractive index by using capillary tubes which have the equal ratio of an inner diameter and an outer diameter and different glass refractive indices in place of changing the mean refractive index by changing the ratio between an inner diameter and an outer diameter of the silica capillary tubes and hence, the requirement as to the number of the kinds of boring instruments can be alleviated and the fabrication cost can be reduced.
It is preferable that the arrangement of sub-medium regions is substantially equivalent to a hexagonal lattice or a quadratic lattice. This is because the fabrication of the optical fiber having such a structure is facilitated due to this arrangement and because particularly with respect to the quadratic lattice, the mean refractive index distribution can be easily set such that the mean refractive index distribution in cross section becomes substantially symmetric about the rotation of 90xc2x0.